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Patents: Chaff in the wind

By BARRY FOX

In the Second World War, aircraft dropped tinsel strips of metal foil,
called chaff, which confused the enemy’s radar by creating a screen which
reflects electromagnetic radiation. European patent application 390 580,
from the American chemical company Du Pont of Wilmington, Delaware, describes
a type of polymer chaff designed to float for longer.

Du Pont proposes using lightweight polymers impregnated with tiny particles
of conductive metal such as iron, copper or aluminium. These metals are
embedded inside the polymer material so that the air cannot oxidise them,
spoiling their effectiveness.

Finely divided metal particles are dispersed in a solution of cellulose
acetate in dimethylacetamide. The solution is then stirred with paddle wheels
set at an angle to create shearing action as the cellulose precipitates.
This forms a web of individual hair-like particles of metal around 1 micrometre
in diameter. The material is then freeze-dried. The new fibres are so light
that they settle through the air at a rate of less than a metre a minute.